arXiv subjects

Yonah Lemonik

Publications and source records attributed to Yonah Lemonik.

8 recordsLinked to original sources

arXiv2019

Transport and spectral signatures of transient fluctuating superfluids in the absence of long-range order

Results are presented for the quench dynamics of a clean and interacting electron system, where the quench involves varying the strength of the attractive interaction along arbitrary quench trajectories. The initial state before the quench is assumed to be a normal electron gas, and the dynamics is studied in a regime where long-range order is absent, but nonequilibrium superconducting fluctuations need to be accounted for. A quantum kinetic equation using a two-particle irreducible formalism is derived. Conservation of energy, particle-number, and momentum emerge naturally, with the conserved currents depending on both the electron Green's functions and the Green's functions for the superconducting fluctuations. The quantum kinetic equation is employed to derive a kinetic equation for the current, and the transient optical conductivity relevant to pump-probe spectroscopy is studied. The general structure of the kinetic equation for the current is also justified by a phenomenological approach that assumes model-F in the Halperin-Hohenberg classification, corresponding to a non-conserved order-parameter coupled to a conserved density. Linear response conductivity and the diffusion coefficients in thermal equilibrium are also derived, and connections with Aslamazov-Larkin fluctuation corrections are highlighted. Results are also presented for the time-evolution of the local density of states. It is shown that Andreev scattering processes result in an enhanced density of states at low frequencies. For a quench trajectory corresponding to a sudden quench to the critical point, the density of states is shown to grow in a manner where the time after the quench plays the role of the inverse detuning from the critical point.

cond-mat.supr-con↗

arXiv2018

Quench dynamics of superconducting fluctuations and optical conductivity in a disordered system

There has been significant interest in the generation of very short-lived superconducting states in solid-state films. Here we consider the role of non-equilibrium superconducting fluctuations in such systems, generated by an interaction quench, considering the limit of large static disorder. In particular, we argue that because of critical slowing down, the regime of the fluctuation dominated \emph{normal state} is more important than might be naïvely thought. We show how such a state might appear in the optical conductivity, and give the appropriate non-equilibrium generalization of the Azlamazov-Larkin and Maki-Thompson fluctuation corrections to the optical conductivity. For a quench to the superconducting critical point, we show that the fluctuation corrections lead to power-law aging behavior in the optical conductivity. The power-law aging from Azlamazov-Larkin and Maki-Thompson terms are qualitatively different, despite the effect of the two being similar in dc measurements in thermal equilibrium. These signatures provide a road-map for experiments to identify the role of superconducting fluctuations in a transient state.

cond-mat.str-el↗

arXiv2018

Central Charge of Periodically Driven Critical Kitaev Chains

Periodically driven Kitaev chains show a rich phase diagram as the amplitude and frequency of the drive is varied, with topological phase transitions separating regions with different number of Majorana zero and π modes. We explore whether the critical point separating different phases of the periodically driven chain may be characterized by a universal central charge. We affirmatively answer this question by studying the entanglement entropy (EE) numerically and analytically for the lowest entangled many particle eigenstate at arbitrary nonstroboscopic and stroboscopic times. We find that the EE at the critical point scales logarithmically with a time-independent central charge, and that the Floquet micromotion gives only subleading corrections to the EE. This result also generalizes to multicritical points where the EE is found to have a central charge that is the sum of the central charges of the intersecting critical lines.

cond-mat.str-el↗

arXiv2018

Model Predictions for Time-Resolved Transport Measurements Made near the Superfluid Critical Points of Cold Atoms and $K_3C_{60}$ Films

Recent advances in ultrafast measurement in cold atoms, as well as pump-probe spectroscopy of $K_3 C_{60}$ films, have opened the possibility of rapidly quenching systems of interacting fermions to, and across, a finite temperature superfluid transition. However, determining that a transient state has approached a second-order critical point is difficult, as standard equilibrium techniques are inapplicable. We show that the approach to the superfluid critical point in a transient state may be detected via time-resolved transport measurements, such as the optical conductivity. We leverage the fact that quenching to the vicinity of the critical point produces a highly time dependent density of superfluid fluctuations, which affect the conductivity in two ways. First, by inelastic scattering between the fermions and the fluctuations, and second by direct conduction through the fluctuations, with the latter providing a lower resistance current carrying channel. The competition between these two effects leads to nonmonotonic behavior in the time- resolved optical conductivity, providing a signature of the critical transient state.

cond-mat.supr-con↗

arXiv2017

Time-resolved spectral density of interacting fermions following a quench to a superconducting critical point

Results are presented for the time evolution of fermions initially in a non-zero temperature normal phase, following the switch on of an attractive interaction. The dynamics are studied in the disordered phase close to the critical point, where the superfluid fluctuations are large. The analysis is conducted within a two-particle irreducible, large $N$ approximation. The system is considered from the perspective of critical quenches where it is shown that the fluctuations follow universal model A dynamics. A signature of this universality is found in a singular correction to the fermion lifetime, given by a scaling form $t^{(3-d)/2}S_d(\varepsilon^2 t)$, where $d$ is the spatial dimension, $t$ is the time since the quench, and $\varepsilon$ is the fermion energy. The singular behavior of the spectral density is interpreted as arising due to incoherent Andreev reflections off superfluid fluctuations.

cond-mat.supr-con↗

arXiv2016

Entanglement properties of Floquet Chern insulators

Results are presented for the entanglement entropy and spectrum of half-filled graphene following the switch on of a circularly polarized laser. The laser parameters are chosen to correspond to several different Floquet Chern insulator phases. The entanglement properties of the unitarily evolved wavefunctions are compared with the state where one of the Floquet bands is completely occupied. The true states show a volume law for the entanglement, whereas the Floquet states show an area law. Qualitative differences are found in the entanglement properties of the off-resonant and on-resonant laser. Edge states are found in the entanglement spectrum corresponding to certain physical edge states expected in a Chern insulator. However, some edge states that would be expected from the Floquet band structure are missing from the entanglement spectrum. An analytic theory is developed for the long time structure of the entanglement spectrum. It is argued that only edge states corresponding to off-resonant processes appear in the entanglement spectrum.

cond-mat.mes-hall↗

arXiv2016

Entanglement properties of the critical quench of O(N) bosons

The entanglement properties of quenched quantum systems have been studied for a decade, however results in dimensions other than $d=1$ are generally lacking. We remedy this by investigating the entanglement properties of bosonic critical systems in $d=3$, both numerically and analytically, comparing the free and the interacting critical quench of an $O(N)$ model. We find that the evolution of the entanglement entropy for these two systems is nearly identical, as expected from the "quasi-particle" picture. However, the low-lying entanglement spectrum is controlled by the different critical exponent of the two systems, and therefore these exponents may be extracted by purely entanglement-theoretic calculations. We verify this scaling numerically.

cond-mat.quant-gas↗

arXiv2014

Quantum Memory Effects in Disordered Systems and Their Relation to 1/f Noise

We propose that memory effects in the conductivity of metallic systems can be produced by the same two levels systems that are responsible for the 1/f noise. Memory effects are extremely long-lived responses of the conductivity to changes in external parameters such as density or magnetic field. Using the quantum transport theory, we derive a universal relationship between the memory effect and the 1/f noise. Finally, we propose a magnetic memory effect, where the magneto-resistance is sensitive to the history of the applied magnetic field.

cond-mat.mes-hall↗